This invention relates generally to radio frequency filter circuits and more particularly to radio frequency transversal and recursive filters.
As is known in the art, a transversal filter, for example, samples a signal at a plurality of intervals of time, weights each one of the individual sampled signals, and sums each one of the individual sampled signals to produce an output signal.
Transversal filters are more common in digital signal processing. However, analog versions of transversal filters are also known. Exemplary of an analog transversal filter of the prior art fabricated from discrete components is shown in U.S. Pat. No. 4,291,268 to Wagner. Wagner describes a transversal filter having an input delay line coupled to an output delay line by a plurality of transversal elements. Both the input and output delay lines are distributed microstrip transmission lines. The lines have low pass characteristics with cutoff frequencies above that of the transversal elements. Field effect transistors are used as the transversal elements to weight signals coupled between the input and output lines. The transistors are successively coupled by the input line and spaced by a phase shift t.sub.a corresponding to a predetermined phase delay. Similarly, the output electrodes are also spaced by a phase shift t.sub.b corresponding to a predetermined phase delay.
An input signal propagates along the input delay line and has portions thereof successively coupled to the transversal elements. Each successive portion has is provided with a successively increasing phase delay given by Nt.sub.a. Likewise output signals from each one of the transistors are successively coupled to an output terminal by the output lines with successive phase delays N(t.sub.a +t.sub.b), where N is the number of phase shift lengths between the input electrodes of the transistors.
In transversal filter theory, it is known that by selecting the values of the weighting factors for the transversal elements, a selected filter response is provided. For example, a bandpass, low pass, or high pass filter response characteristic may selectively be provided with such an arrangement.
To provide a bandpass characteristic having steep or sharp frequency cutoffs at the band edges in the Wagner filter and those of the prior art, it is generally required to have a large number of transversal elements typically in excess of 20-26. This follows in the Wagner circuit because the microstrip transmission lines have cut-off frequencies substantially above the frequency capabilities of the transistors. Thus the transmission lines do not contribute to the basic shape of the frequency passband of the filter.
When one considers that the phase delays t.sub.a and t.sub.b in the Wagner filter are related to a multiple of a quarter of a wavelength at a frequency of operation of the filter, it becomes apparent that to provide a bandpass characteristic having very steep band edge slopes may require 25.times..lambda./4 or 6.25 .lambda. lengths each of input and output delay transmission line length. That is, a 25 element transversal filter will have input delay and output delay lines equal to 6.25 wavelengths each for a total of 12.5 wavelengths at the nominal frequency of the filter.
In gallium arsenide, for example, a preferred material for fabrication of monolithic microwave integrated circuits, a wavelength at 10 GHz equals approximately 11 millimeters. Accordingly, at 10 GHz such a transversal filter would have a length equal to approximately 132 millimeters. Such a circuit is extremely large for a monolithic microwave integrated circuit. Furthermore, since each chip would be large in size, the number of chips which may be processed per wafer is concomitantly reduced, and the cost of such a circuit would be relatively high. One of the advantages of monolithic microwave integrated circuits is the ability to fabricate large numbers of identical circuits on a single wafer in one fabrication step. This advantage is reduced by very large chip sizes. Furthermore, the yield of good devices from any particular wafer would also be concomitantly reduced, due to the lower number of circuits per wafer and the requirement to have correct processing occur over a large area to provide a single complete, operable device. Electrically, the lengths of the transmission lines also pose problems for the microwave designer. The 132 mm length lines would tend to be extremely lossy, requiring complex compensation to maintain the designed for coefficients for the weighting factors. Furthermore, a very large integrated circuit, as would be expected from this technique, would be difficult to use in certain applications of these circuits. One application of a transversal filter would be as part of a phased array network for a phased array antenna. Extremely large circuits as indicated above would make their use in a phased array network extremely difficult, since the elements of the phased array network are generally spaced approximately .lambda./4 to .lambda./2 (free-space) wavelengths apart. Therefore, packaging of these circuits in such a phased array network with other required phased array components would be extremely difficult.
A second problem with the conventional transversal filter as exemplified by Wagner is that as a bandpass filter harmonic passbands are also present. The harmonic passbands are produced because the essential band characteristics of the microstrip transmission lines which provide input and output lines are as low pass filters having very high cut-off frequencies beyond the desired passband of the filter. The circuit is designed such that over a given frequency band, the signals components through each element add up in some predetermined phase relationship. However, at harmonics of this frequency band, this relationship is also true. Therefore, at each harmonic of the transversal filter, there would be a corresponding passband. In certain applications of transversal filters, such as an electronic surveillance measures systems, the presence of harmonic passbands makes processing of received signals more difficult or requires the use of additional filtering to remove the harmonic passbands.
A recursive filter is similar to a transversal filter except that the recursive filter includes recursive elements, that is elements which sample the output signal over intervals of time and feed the output signal back to the input side of the filter. If a recursive filter is fabricated in the same manner as that described in the Wagner patent by use of recursive elements, the problems associated with the transversal filter will equally apply to the recursive filter. Recursive filters due to feedback are generally smaller than transversal filters. The structure described by Wagner does not easily apply to recursive elements since it would require two field effect transistors connected in two opposite directions. The resulting feedback from this particular arrangement may be highly unstable.
Accordingly, it is desirable to provide an analog transversal or analog recursive filter which has the desirable frequency slope characteristics as provided when a large number of transversal and/or recursive elements are used but without the large space requirement. Furthermore, it is also desirable to provide a transversal and/or recursive bandpass filter which does not have harmonic passbands.